Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 77738817c2a7d27fd5808bd06158f47586f5032b..2668a8fb122a8818d7fef38273e4be079f8197fc 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -990,11 +990,12 @@ class JavaScriptBackend extends Backend {
   }
 
   void registerIsCheck(DartType type, Enqueuer world) {
-    if (!type.isRaw) {
+    if (!type.isRaw || type is TypeVariableType) {
       enqueueInResolution(getSetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeArgument());
       enqueueInResolution(getCheckArguments());
+      enqueueInResolution(getGetObjectIsSubtype());
     }
     // [registerIsCheck] is also called for checked mode checks, so we
     // need to register checked mode helpers.
@@ -1427,6 +1428,10 @@ class JavaScriptBackend extends Backend {
     return compiler.findHelper(const SourceString('checkArguments'));
   }
 
+  Element getGetObjectIsSubtype() {
+    return compiler.findHelper(const SourceString('objectIsSubtype'));
+  }
+
   Element getThrowNoSuchMethod() {
     return compiler.findHelper(const SourceString('throwNoSuchMethod'));
   }